STEREONET BASICS Pages 692-704 (The figures in this section of

STEREONET BASICS
Pages 692-704
(The figures in this section of
your text are especially
important)
Stereonets
• Stereonets are used for plotting and
analyzing 3-D orientations of lines and
planes in 2-D space
• It is MUCH more convenient than using
Cartesian space (x-y-z coordinates) for
graphically representing and analyzing
3-D data
Stereonets: Why bother?
Stereonets are used in:
• Landslide hazard/slope failure studies
• Earthquake studies
• Fracture analyses used in
hydrogeology and/or groundwater
pollution potentials
• Mining industry (fossil fuels included)
• Engineering
• Practically anything that deals with
relative orientations of planes and lines
Mysteries of Stereographic projection
(691-694)
• Any line or plane can be assumed to pass
through the center of a reference sphere
• Planes intersect the lower hemisphere as
GREAT CIRCLES
• Lines intersect the lower hemisphere as
POINTS
• The great circles or points are projected
on the horizontal plane to create
STEREOGRAPHIC PROJECTIONS or
stereograms
Small circles
(Look like LATITUDES)
Great circles
(Look like
LONGITUDES)
Mysteries of Stereographic projection
(691-694)
• The horizontal plane or the plane of
reference (the EQUATORIAL PLANE,
Page 692) is represented by the outer
circle of the stereogram
• A vertical plane shows up as a straight
line on the stereogram
• Inclined planes (0<dip angle<90º) are
represented by projections of the great
circles (show up as curved lines)
Equatorial
circle =
horizontal
plane
Straight lines =
vertical planes
0
20
40
60
80
80
60
40
20
0
Dip
angles
Great circles
= inclined
planes
Mysteries of Stereographic projection
(691-694)
• The projection of a gently dipping plane
(dip angle <45º) will be more curved than
that of a steeply dipping plane (dip angle
> 45º)
• A line is represented as a point on the
stereogram
• A horizontal line will project as a point on
the equatorial plane
• Vertical line???
N
20
40
Small circles = Paths of inclined
lines around the N-S axis
60
80
W
E
S
Lines and planes are plotted as
stereograms by combining the great
and the small circles on the
stereonets
Plotting the orientation of a line
using a stereonet (694-697)
Lab 2
Plotting a plane by its dip and dip direction
on a stereonet (also known as DIP VECTOR)
• Dip = inclination of the line of greatest
slope on an inclined plane
• Refers to TRUE DIP as opposed to
APPARENT DIP of a plane
• 0 ≤ apparent dip <true dip
• Dip direction is ALWAYS perpendicular
to strike direction
• The dip and dip direction of an inclined
plane completely defines its attitude
• Plotted the same way as lines
Defining a plane by its POLE (page 698)
• POLE of a plane = line perpendicular to
the plane
• A plane can have ONLY ONE pole
• The orientation of the pole of a plane
completely defines the orientation of
the plane
• This is the MOST common way planes
are represented on a stereogram
Plotting the pole of a plane (page 698)
• If you have strike/dip/dip direction data,
Start the same way you normally would
for plotting the great circle for the plane
• Identify the dip line (the line of greatest
slope) on the great circle ***
• The POLE is the line perpendicular to the
dip line
• To get to the pole of the plane, count 90
from the dip line along the E-W vertical
plane, and mark the point
***You don’t need to draw the great circle
Measuring the angle between two lines
Angle between two lines is measured on
the plane containing both lines
• Plot the points representing the lines
• Rotate your tracing paper so both
points lie on the same great circle.
This great circle represents the plane
containing both lines
• Count the small circles between those
two points along the great circle to
determine the angle between the lines.
Measuring the angle between two planes
• Angle between two planes is the same as
the angle between their poles (this is yet
another reason for plotting poles instead
of great circles for planes)
• Plot the poles for the planes
• Rotate your tracing paper so both poles
lie on the same great circle.
• Count the small circles between those
two poles along the great circle to
determine the angle between the two
planes.
Measuring angle between two planes
on stereonet (lab 3)
Measure the angles between the pairs of planes with the given attitudes
Pair #1
Strike
Dip/dip direction
1. 342
38NE
2. S27W 43SE
Pair #2
1. N35W 57SW
2. 278
23N
Pair #3
1. 132
2. N25E
65SW
71NW
Plotting a plane using trend and plunge
(or apparent dip/dip direction) data of
two lines lying on that plane
• Plot the points representing the lines
• Rotate your tracing paper so those two
points lie on the same great circle
• Trace and label that great circle
Plotting a plane from trend/plunge data
of two lines (lab 3)
Identify the plane containing the following pairs of lines with the given attitudes
Trend
1. 357.5
2. 112.5
Plunge
67
26
Pair #2
1. 17.5
2. 282.5
58
59
Pair #3
1. 77.5
2. 330.5
90
58
Pair #1